Keith Waclena has a great Introduction to GNU Emacs. It’s an ongoing project that started back with GNU Emacs 19.29 in 1997 and has been in continual development since. This latest version is for Emacs 28.2. Waclena works for the University of Chicago as an applications systems analyst but his tutorial reads like (in a good sense) it was written by a professor trying to help his students learn Emacs.
His view of Emacs is much like mine: it’s not really an editor but rather “the last remaining Lisp Machine”. This means, among other things, it can be usefully though of as an operating system in that there are applications to perform almost any task that you need to perform.
It’s pretty clear that Waclena intended this to be a book or at least book-like. In several places he refers to it as a book and it is pretty comprehensive. He even covers such esoteric matters as dealing with the Meta key, how many spaces there should be after a period, Emacs versus the Unix philosophy, and usual myths about Emacs being a resource hog and slow.
This is a large document that covers at lot of material. It’s the best introduction to Emacs for beginners that I’ve seen. The document is also available in PDF or EPUB formats. If you’re looking for a comprehensive introduction to Emacs, this is a good place to start. Some of the topics are still TBD—as befits a work in progress—so it will doubtless continue to improve. It’s worth a look even if you’re an experienced Emacser.